1. Field of Invention
The invention relates to techniques used for generating or deriving single frequency signals as may be required in communication systems, as for example for pilot carrier frequencies, signaling tones, and the like; and more particularly to means and methods by which such signals may be derived from available logic signals of different frequencies.
2. Description of Prior Art
Heretofore, single frequency signals of the character described have been obtained by subdividing the frequency of a clock source, as by means of a counter, or by modulation wherein a beat signal is obtained having a frequency equal to the sum or difference of the frequencies of the two more basic signals used in the modulation process. The frequency division technique may be used only where the frequency division requirement corresponds with an available counter division ratio. Thus, given say a 720 kHz clock source, an 8 kHz signal may be obtained by using a divide-by-90 counter. On the other hand, if there was available in the system say a 160 kHz signal and a 72 kHz signal was required, derivation of the latter by a division technique is not feasible. However, if in the same system an 8 kHz signal is available, the 160 kHz signal may be reduced to an 80 kHz signal by a divide-by-2 counter and then modulated with the 8 kHz signal to produce the required 72 kHz signal.
The usual method for obtaining the product P(t) = sin 2.pi.(f.sub.m .+-. f.sub.n)t is accomplished by the well-known process of modulation wherein the frequency of a single frequency wave (normally called the carrier wave) is varied in step with the instantaneous value of a second signal, called the modulating wave. The signals to be modulated A(t) = sin 2.pi.f.sub.m t and B(t) = sin.pi.2 f.sub.n t may be either sine waves as indicated or square waves. These are fed into a modulator usually made up of diodes and/or transistors and quite often transformers; and a filter is placed at the output of the modulator to select the desired output signal sin 2.pi.(f.sub.m + f.sub.n) or sin 2.pi.(f.sub.m - f.sub.n)t. In equipment which uses mainly integrated digital logic circuits it is advantageous to use standard logic circuits whenever possible and to minimize the use of the discrete, analog components such as required by the usual modulator.